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diff --git a/man7/time.7 b/man7/time.7 deleted file mode 100644 index 7259feb..0000000 --- a/man7/time.7 +++ /dev/null @@ -1,218 +0,0 @@ -.\" Copyright (c) 2006 by Michael Kerrisk <mtk.manpages@gmail.com> -.\" -.\" SPDX-License-Identifier: Linux-man-pages-copyleft -.\" -.\" 2008-06-24, mtk: added some details about where jiffies come into -.\" play; added section on high-resolution timers. -.\" -.TH time 7 2023-10-31 "Linux man-pages 6.7" -.SH NAME -time \- overview of time and timers -.SH DESCRIPTION -.SS Real time and process time -.I "Real time" -is defined as time measured from some fixed point, -either from a standard point in the past -(see the description of the Epoch and calendar time below), -or from some point (e.g., the start) in the life of a process -.RI ( "elapsed time" ). -.P -.I "Process time" -is defined as the amount of CPU time used by a process. -This is sometimes divided into -.I user -and -.I system -components. -User CPU time is the time spent executing code in user mode. -System CPU time is the time spent by the kernel executing -in system mode on behalf of the process (e.g., executing system calls). -The -.BR time (1) -command can be used to determine the amount of CPU time consumed -during the execution of a program. -A program can determine the amount of CPU time it has consumed using -.BR times (2), -.BR getrusage (2), -or -.BR clock (3). -.SS The hardware clock -Most computers have a (battery-powered) hardware clock which the kernel -reads at boot time in order to initialize the software clock. -For further details, see -.BR rtc (4) -and -.BR hwclock (8). -.SS The software clock, HZ, and jiffies -The accuracy of various system calls that set timeouts, -(e.g., -.BR select (2), -.BR sigtimedwait (2)) -.\" semtimedop(), mq_timedwait(), io_getevents(), poll() are the same -.\" futexes and thus sem_timedwait() seem to use high-res timers. -and measure CPU time (e.g., -.BR getrusage (2)) -is limited by the resolution of the -.IR "software clock" , -a clock maintained by the kernel which measures time in -.IR jiffies . -The size of a jiffy is determined by the value of the kernel constant -.IR HZ . -.P -The value of -.I HZ -varies across kernel versions and hardware platforms. -On i386 the situation is as follows: -on kernels up to and including Linux 2.4.x, -HZ was 100, -giving a jiffy value of 0.01 seconds; -starting with Linux 2.6.0, -HZ was raised to 1000, -giving a jiffy of 0.001 seconds. -Since Linux 2.6.13, the HZ value is a kernel -configuration parameter and can be 100, 250 (the default) or 1000, -yielding a jiffies value of, respectively, 0.01, 0.004, or 0.001 seconds. -Since Linux 2.6.20, a further frequency is available: -300, a number that divides evenly for the common video frame rates -(PAL, 25 Hz; NTSC, 30 Hz). -.P -The -.BR times (2) -system call is a special case. -It reports times with a granularity defined by the kernel constant -.IR USER_HZ . -User-space applications can determine the value of this constant using -.IR sysconf(_SC_CLK_TCK) . -.\" glibc gets this info with a little help from the ELF loader; -.\" see glibc elf/dl-support.c and kernel fs/binfmt_elf.c. -.\" -.SS System and process clocks; time namespaces -The kernel supports a range of clocks that measure various kinds of -elapsed and virtual (i.e., consumed CPU) time. -These clocks are described in -.BR clock_gettime (2). -A few of the clocks are settable using -.BR clock_settime (2). -The values of certain clocks are virtualized by time namespaces; see -.BR time_namespaces (7). -.\" -.SS High-resolution timers -Before Linux 2.6.21, the accuracy of timer and sleep system calls -(see below) was also limited by the size of the jiffy. -.P -Since Linux 2.6.21, Linux supports high-resolution timers (HRTs), -optionally configurable via -.BR CONFIG_HIGH_RES_TIMERS . -On a system that supports HRTs, the accuracy of sleep and timer -system calls is no longer constrained by the jiffy, -but instead can be as accurate as the hardware allows -(microsecond accuracy is typical of modern hardware). -You can determine whether high-resolution timers are supported by -checking the resolution returned by a call to -.BR clock_getres (2) -or looking at the "resolution" entries in -.IR /proc/timer_list . -.P -HRTs are not supported on all hardware architectures. -(Support is provided on x86, ARM, and PowerPC, among others.) -.SS The Epoch -UNIX systems represent time in seconds since the -.IR Epoch , -1970-01-01 00:00:00 +0000 (UTC). -.P -A program can determine the -.I "calendar time" -via the -.BR clock_gettime (2) -.B CLOCK_REALTIME -clock, -which returns time (in seconds and nanoseconds) that have -elapsed since the Epoch; -.BR time (2) -provides similar information, but only with accuracy to the -nearest second. -The system time can be changed using -.BR clock_settime (2). -.\" -.SS Broken-down time -Certain library functions use a structure of -type -.I tm -to represent -.IR "broken-down time" , -which stores time value separated out into distinct components -(year, month, day, hour, minute, second, etc.). -This structure is described in -.BR tm (3type), -which also describes functions that convert between calendar time and -broken-down time. -Functions for converting between broken-down time and printable -string representations of the time are described in -.BR ctime (3), -.BR strftime (3), -and -.BR strptime (3). -.SS Sleeping and setting timers -Various system calls and functions allow a program to sleep -(suspend execution) for a specified period of time; see -.BR nanosleep (2), -.BR clock_nanosleep (2), -and -.BR sleep (3). -.P -Various system calls allow a process to set a timer that expires -at some point in the future, and optionally at repeated intervals; -see -.BR alarm (2), -.BR getitimer (2), -.BR timerfd_create (2), -and -.BR timer_create (2). -.SS Timer slack -Since Linux 2.6.28, it is possible to control the "timer slack" -value for a thread. -The timer slack is the length of time by -which the kernel may delay the wake-up of certain -system calls that block with a timeout. -Permitting this delay allows the kernel to coalesce wake-up events, -thus possibly reducing the number of system wake-ups and saving power. -For more details, see the description of -.B PR_SET_TIMERSLACK -in -.BR prctl (2). -.SH SEE ALSO -.ad l -.nh -.BR date (1), -.BR time (1), -.BR timeout (1), -.BR adjtimex (2), -.BR alarm (2), -.BR clock_gettime (2), -.BR clock_nanosleep (2), -.BR getitimer (2), -.BR getrlimit (2), -.BR getrusage (2), -.BR gettimeofday (2), -.BR nanosleep (2), -.BR stat (2), -.BR time (2), -.BR timer_create (2), -.BR timerfd_create (2), -.BR times (2), -.BR utime (2), -.BR adjtime (3), -.BR clock (3), -.BR clock_getcpuclockid (3), -.BR ctime (3), -.BR ntp_adjtime (3), -.BR ntp_gettime (3), -.BR pthread_getcpuclockid (3), -.BR sleep (3), -.BR strftime (3), -.BR strptime (3), -.BR timeradd (3), -.BR usleep (3), -.BR rtc (4), -.BR time_namespaces (7), -.BR hwclock (8) |